拉氏变换
(包含盘算公式)
拉氏变换及反变换公式1. 拉氏变换的基本性质
1 L[af(t)],aF(s)齐次性 线性定理 L[f(t),f(t)],F(s),F(s)1212叠加性 dft(),,LsFsf[]()(0) dt 2dft()2,,, ,LsFssff()[]()(0)0 2dt ?
n2 微分定理 一般形式 ndft()nn,k(k,1),,,,LsFssf()(0)n,dtk,1k,1dft()(k,1),ft()k,1dt
n初始条件为0时 df(t)n L[],sF(s)ndt
[f(t)dt]F(s)t,0,L[f(t)dt],,, ss
2 [f(t)dt][f(t)(dt)]F(s)tt,0,0,,,2L[f(t)(dt)],,,,,22 一般形式 sss
?3 积分定理 共个共个nn,,nF(s)1nnL[?f(t)(dt)],,[?f(t)(dt)],t,0nnk,,1,,,,ssk,1
n共个,F(s)n 初始条件为0时 L[f(t)(dt)],?n,,s
,Ts L[f(t,T)1(t,T)],eF(s)4 延迟定理(或称域平移定理) t
,at 5 衰减定理(或称域平移定理) L[f(t)e],F(s,a)s
limf(t),limsF(s)6 终值定理 t,,s,0
limf(t),limsF(s) 7 初值定理 t,0s,,
tt L[f(t,,)f,()d,],L[f(t)f(t,,)d,],F(s)F(s)8 卷积定理 121212,,00
2( 常用函数的拉氏变换和z变换表
序
拉氏变换E(s) 时间函数e(t) Z变换E(z) 号
1 δ(t) 1 1
,1z ,(t),,(t,nT) ,T2 ,Tsz,1n0,1,e
1z 1(t) 3 z,1s
Tz1 t 4 22(z,1)s
22Tz(z,1)1t 5 332(z,1)s2
nnn1(,1),zt lim()6 ,naTn,1a,0n!,az,en!s
1z,at e 7 ,aTs,az,e
,aT1Tze,at te 8 2,aT2(s,a)(z,e)
,aTa(1,e)z ,at 1,e 9 ,aT(z,1)(z,e)s(s,a)
b,azz,at,bt , e,e 10 ,aT,bT(s,a)(s,b)z,ez,e
,,zsinT sin,t 11 222s,,z,2zcos,T,1
,sz(z,cosT) cos,t12 222z,2zcos,T,1s,,
,aT,,zesinT,at 13 22 esin,t2,aT,2aT(s,a),,z,2zecos,T,e
2,aTs,a,z,zecosT ,at 2214 ecos,t2,aT,2aT(s,a),,z,2zecos,T,e
1z t/T a 15 s,(1/T)lnaz,a
3( 用查表法进行拉氏反变换
用查表法进行拉氏反变换的关键在于将变换式进行部分分式展开,然后逐项查表进行反变换。
F(s)设是的有理真分式 s
1mm,bs,bs,?,bs,bB(s)110mm,F(s),, () n,m1nn,A(s)as,as,?,as,a110nn,
F(s)式中系数,都是实常数;是正整数。按代数定理可将展m,na,a,...,a,ab,b,?b,b01n,1n01m,1m
开为部分分式。分以下两种情况讨论。
? 无重根 A(s),0
这时,F(s)可展开为n个简单的部分分式之和的形式。
nccccc12iniFs,,,,,,, ()??,s,ss,ss,ss,ss,s,1i12ini式中,是特征方程A(s),0的根。为待定常数,称为F(s)在处的留数,可按下式计ss,s,?,scii12n
算:
c,lim(s,s)F(s)ii,ssi
或
B(s) c, i,A(s)s,si
,式中,为对的一阶导数。根据拉氏变换的性质,从式(F-1)可求得原函数sA(s)A(s)
nn,,c,1,1,stiiftL,,FsL(),(),ce , ,,i,,ss,i,1i,1i,,
? 有重根 A(s),0
设有r重根,F(s)可写为 sA(s),01
B(s)Fs,,, r(s,s)(s,s)?(s,s)1r,1n
ccccccinrr,11r,1,,,,,,,,=???rr,1(s,s)(s,s)(s,s)s,ss,ss,s111r,1in
式中,为F(s)的r重根,,…, 为F(s)的n-r个单根; sssn1r,1
其中,,…, 仍按式(F-2)或(F-3)计算,,,…, 则按下式计算: cccccnr,1rr,11
r c,lim(s,s)F(s)1r,ss1
dr c,lim[(s,s)F(s)],11rds,ss1
?
j()1dr (F-5)c,lim(s,s)F(s)rj1,j()ss,1j!ds
?
(r,1)1dr c,lim(s,s)F(s)11(r1),ss,1(r,1)!ds
f(t)原函数为
,1 f(t),L,,F(s)
,,cccccc,1inrr,11r,1 L???,,,,,,,,,,,rr,1(ss)ssssss,,,,(ss)(ss),,1r,1in11,,
ncc,,ststrr,1,2rr,1i1,t,t,,ct,ce,ce? (F-6),i21,,(r,1)!(r,2)!ir,,1,,